Esposito has been on hiatus since October when she requested a "very limited part-time schedule" to allow her time to deal with her Celiac disease. Esposito claims that the network knew the reasoning behind her request, but put her and her character on leave regardless as the role couldn't handle the demand of a part-time schedule - this, she says, resulted in her being unable to work elsewhere during her "unpaid leave."
News that Esposito will not return to the show has not yet been officially confirmed by the network or Blue Bloods producers, but sources close to the show reportedly say that it's a done deal.
In recent episodes, new characters portrayed by Megan Ketch and Megan Boone have been rotated to substitute for Esposito's absence.
[via TV Line]